Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output and a flow of exhaust gases. The exhaust gases can include a complex mixture of air pollutants produced as byproducts of the combustion process. For example, an engine can produce NOx, particulate matter, and hydrocarbons. Due to increased attention on the environment, the amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of exhaust emissions includes exhaust gas recirculation (EGR). EGR is the recirculation of a portion of the exhaust gas produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture, when ignited, produces a lower combustion temperature and a corresponding reduced amount of NOx.
An exemplary EGR system is disclosed in U.S. Patent Application Publication No. US 2012/0222659 A1 to Kulkarni et al. that published on Sep. 6, 2012 (“the '659 publication”). The '659 publication discloses a four-stroke engine having a plurality of donor cylinders and a plurality of non-donor cylinders. Exhaust gases from the non-donor cylinders are directed to a first exhaust manifold, which routes the exhaust gases through a turbine and to the atmosphere. Exhaust gases from the donor cylinders are directed to a second exhaust manifold, which routes the exhaust gases through an exhaust gas recirculation (EGR) system and into an intake passage for both the donor and non-donor cylinders. The EGR system includes an EGR cooler to cool the exhaust gases before the exhaust gases return to the intake passage. The donor and non-donor cylinders are positioned in two banks of cylinders, with some donor cylinders arranged in between non-donor cylinders along each of the two banks of cylinders. In addition, two or more of the donor cylinders may be positioned immediately adjacent one another at a middle point along one of the two banks of cylinders, in order to reduce engine noise and vibration and to reduce a size of the second exhaust manifold, which routes exhaust gas from the donor cylinders to the intake passage of the engine.
Although the system of the '659 publication may help lower engine emissions by re-circulating the exhaust to the intake passage of the engine, the system may still be less than optimal. Specifically, the system of the '659 publication may be applicable to four-stroke engines. Two-stroke engines, which do not have discrete intake and exhaust strokes, may experience problems with pumping the exhaust from the donor cylinders back into the intake passage of the engine. Additionally, arranging the donor cylinders at locations in between the non-donor cylinders along the bank of cylinders may increase the size of the exhaust manifold associated with the donor cylinders and cause problems with packaging other components associated with the EGR system within the engine system.
One exemplary EGR system for a two-stroke engine is disclosed in U.S. Patent Application Publication No. US 2013/0081392 A1 to Klingbeil that published on Apr. 4, 2013 (“the '392 publication”). The '392 publication discloses a two-stroke engine having a plurality of donor cylinders and a plurality of non-donor cylinders. The engine also includes two compressors connected to a single turbine via a single shaft. The compressors supply air to the donor cylinders and the non-donor cylinders, respectively. Exhaust generated in the donor cylinders is routed back to mix with the air supplied to the non-donor cylinders. The '392 publication also discloses a bypass valve allowing some of the compressed air from the donor cylinders to mix with the compressed air supplied to the non-donor cylinders. This configuration can assist in reduction of EGR rate to the non-donor cylinders and assist in over-boosting of the donor cylinders.
While the system of the '392 publication can help control the EGR rate, the system may still be less than optimal. Specifically, by having two compressors connected to a single turbine, the system of the '392 publication may experience problems during start up and when transitioning to higher loads. In addition, the bypass valve of the '392 publication allows some of the compressed air from the donor cylinders to mix with the compressed air supplied to the non-donor cylinders, but does not allow some of the compressed air from the non-donor cylinders to mix with the compressed air supplied to the donor cylinders. In some situations (e.g., during start up and when transitioning to higher loads), it may be desirable to allow some of the compressed air from the non-donor cylinders to mix with the compressed air supplied to the donor cylinders.
The engine system of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.